1. Field of the Invention
The present invention relates to oxymethylene copolymers. It relates more particularly to copolymers containing oxymethylene repeat units with improved heat stability. It also relates to a process for the preparation of these copolymers.
2. Background of the Art
Polyoxymethylenes or polyacetals containing chains of --CH.sub.2 O-- repeat units have been known for many years. They can be manufactured by the polymerization of anhydrous formaldehyde or of its cyclic trimer, trioxane.
High molecular weight homopolymers of polyoxymethylene generally have very good mechanical properties, a wide range of service temperatures, and good resistance to chemical agents, which make them a material of choice for the manufacture of moulded articles which find applications in the fields of precision engineering, the automotive industry, the electrical industry and domestic electrical appliances. However, these homopolymers are characterized by a relative instability to heat, which rsults in their decomposition with the formation of formaldehyde, when they are exposed to high temperatures for prolonged periods, especially during their fabrication.
Attempts have already been made to overcome this disadvantage by manufacturing modified polyoxymethylenes containing other oxyalkylene groups in combination with the oxymethylene groups. Such modified polyoxymethylenes, whose heat stability is higher than that of the corresponding homopolymer, have been obtained by copolymerizing trioxane with other cyclic ethers such as, for example, ethylene oxide, dioxane and dioxolane (French Pat. No. 1,221,148 to Celanese Corp.).
Copolymers with high heat stability, containing oxymethylene and 2-chloromethoxyethylene repeat units have also been prepared by copolymerization of trioxane with a comonomer chosen from epichlorohydrin and 4-chloromethyl-1,3-dioxolane (U.S. Pat. No. 3,072,609).